Vulnerability · Published Jul 1, 2026 · Assigned by runZero
CVE-2026-6686FatFs Use of Uninitialized Clusters After Seek Past EOF
Medium 4.6
In plain English
A medium-severity security flaw in FatFs from ChaN. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 17% of all known vulnerabilities).
Severity (CVSS 3.1)4.6Medium
Exploit odds (30 days)0.3%top 82.8% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known
Official description
FatFs R0.16 and earlier contains an uninitialized cluster exposure when f_lseek() extends files beyond EOF without zero-filling newly allocated clusters. This maps to CWE-908 (Use of Uninitialized Resource). Estimated CVSS v3.1 vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N (4.6, Medium). The estimated CISA SSVC vectors are Exploitation: PoC, Technical Impact: Partial.
Affected products (as reported)
Weakness: CWE-908 · Vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
References
- https://www.runzero.com/blog/fatfs-bugs/ third-party-advisory
- https://github.com/runZeroInc/vulns-2026-fatfs-chance technical-description
- https://elm-chan.org/fsw/ff/ product
- https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/ third-party-advisory